Interstranded DNA cross-links are caused by the clinical drug psoralen and also by some carcinogens and antineoplastic agents. Cells are able to repair cross-linking damages in their DNA, and the complex repair sequence apparently requires a genetic recombinational event between homologous chromosomes. We will study the repair and recombination of cross-linked DNA in bacteria, phage, and animal cells in tissue culture using a variety of biochemical, biophysical and genetical techniques. DNA labeled with radioactive and density iosotopes will be isolated from cells at various stages of repair. Structural features will be characterized by sedimentation and buoyant density properties, sensitivity to DNAases with specific substrate requirements and electron microscopy. Similar experiments will be conducted in permeabilized cells and partial cell systems, as well as with purified DNA substrates and repair enzymes in vitro. The replication of cross-linked DNA will be examined in bacteria and animal cells which are unable to excise cross-links normally. Whether cross-links pass through the growth fork will be determined by isolating and characterizing DNA structures which have recently undergone replication. The chemistry and photobiology of the psoralen reaction with DNA will be studied. Rates of reaction with DNAs of known base composition will establish specific base sequences to which psoralens preferentially bind. The chemical structure of the psoralen cross-linking product in DNA will also be determined.